Very likely the XL6001 is going into thermal shutdown.
The datasheet does not specify a max power, it just says "internally regulated".
and:
- Built in Thermal Shutdown Function
- Built in Current Limit Function
The datasheet does not specify its drive capabilities other than:
The XL6001 could directly drive 5~8 Series 1W LED units at VIN>12V.
- When operating correctly, what is the measured LED voltage from the LED driver output?
- What is the max forward voltage for each LED from the datasheet?
- Are the LEDs are connect in series?
- Is the measured current through the LEDs 636mA?
- XL6001 is configured as only a Boost Driver?